June 11, 2021
Living Hinges Design in 3D Printing - Makenica 3D Printing
This article will discuss the advantages of using living hinges and present design rules and material recommendations when using 3D Printing
Living hinges are a simple, low-cost method of connecting two rigid plastic parts with a flexible joint. This article will discuss the advantages of using living hinges and present design rules and material recommendations when using Online 3D Printing India to produce living hinges.
What are living hinges?
In 3D Printing service in India, a living hinge is a thin flexible web of plastic that connects two or more rigid sections. Typically the larger rigid sections and the living hinge will be made of one continuous piece of plastic. The low cost and simplicity of living hinges make them a popular option for many applications of online 3D Printing Bangalore.
They can be found on almost every commercial product: from drink and shampoo bottles to workshop storage containers and food packaging. Living hinges and the connected rigid sections are manufactured almost exclusively as one part via injection molding.
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Advantages of living hinges
Living hinges are an effective solution of online 3D Printing India when two rigid sections need to be joined together. Some of the advantages of a living hinge include:
Low Cost – Because of their simplicity, living hinges are usually a much cheaper alternative compared to other hinge types.
Durability – Living hinges are specifically designed to be opened repeatedly. They experience very little friction when opened and closed and this typically results in a long lifespan.
Reduced inventory – Living hinges are integrated into a part eliminating the need for extra components in 3D Printing Service in India.
Appearance – Compared to other connection options (assembled hinges, snap-fit connections, etc), living hinges are an aesthetically pleasing and non-obtrusive connection solution.
The main limitation of living hinges centres around their inability to bear any load.
3D printed living hinges
While injection molded living hinges are designed to withstand thousands of cycles without breaking, the nature of 3D printed parts (usually anisotropic, brittle, layer-by-layer constructions) means that 3D printed living hinges are typically used for prototyping or proof-of-concept models where a small number of cycles are needed.
This makes 3D printed living hinges best suited for the verification of a design before needing to invest in expensive injection molding tooling.
The main benefits of 3D printing a living hinge are:
- No need for expensive tooling.
- The design does not need to incorporate features essential to injection molded parts such as gates, runners, or sprues.
- Designs can easily be altered and iterated to achieve an optimal design.
- 3D printing online is able to produce parts quickly further accelerating the design process.
Designing living hinges for 3D printing service
As with other 3D printed features, performance will vary based on design, material, printer calibrations, and layer thickness. Because of this, creating the optimal living hinge for a specific design and technology is often an iterative process. This section offers several design recommendations that can be used as a starting point.
Print direction
Due to the additive, layer-by-layer nature of Online 3D Printing Bangalore, the parts that are produced are typically anisotropic (especially when printing with FDM). To ensure the performance of a living hinge, parts should be orientated so that the width of the hinge rather than the length is built up one layer at a time (the central axis of the hinge should be orientated in the Z-direction). This will often mean printing the part in the vertical build direction.
Hinge geometry
For most prototyping applications, simply printing a thin strip of material is adequate if the hinge is only required to be functional for a few cycles. If a greater number of cycles is required the hinge geometry should be optimized. A well-designed living hinge has a longer outer surface to account for tensile stresses when opening and closing the hinge
As a hinge is closed, it is subjected to bending: the outer surface is placed under tension (and stretches), while the inner surface is compressed. To account for this, a good living hinge design should have a long, curved length on the outer surface and a short inner surface.
For online 3D printing India, more material and a stiffer hinge is generally required to improve the number of cycles before failure. Note though that increasing the thickness of the hinge will also increase the tensile stresses that the outer surface is subjected to.
Design by technology
The materials and processes that each technology of 3D printing service in India produce parts can vary significantly. Because of this, different design rules often apply to each technology. The Online 3D printing Bangalore processes listed below are the most suitable for creating living hinges.
Fused Deposition Modelling (FDM)
The optimal design for a living hinge produced via FDM online 3D Printing India is to try and print the hinge with a single strand of thermoplastic integrated into the rigid sections of the build. The living hinge should be printed in a single strand of thermoplastic to improve strength
This may mean that the part will be printed vertically, resulting in a large amount of required support structure. This will add to the cost and build time of the part. Some dual extrusion FDM printers of online 3D Printing India offer the option to print the hinge section in a secondary flexible material (like TPU) which will further improve hinge performance and the number of cycles before failure.
Build orientation is still important for these materials. Recommended hinge specifications: Minimum of 2-layer thicknesses with 0.4 – 0.8mm recommended
Selective Laser Sintering (SLS)
While SLS parts are less susceptible to delamination of layers when compared to FDM, the build direction is still an important factor when designing living hinges with this 3D printing service in India. SLS parts are usually printed in nylon (PA 12) and the produced hinges typically last around 30 – 50 cycles before failure. Recommended hinge specifications: 0.3 – 0.8mm thick and a minimum of 5 mm in length
Material Jetting
Parts produced via Material Jetting are typically more isotropic than either FDM or SLS parts. The material jetted parts are very smooth and are often aesthetically comparable to injection molded parts. The general rigid photopolymers used in Material Jetting are brittle and unsuitable for prototypes, where more than 10 cycles are required.
One of the major advantages of Material Jetting though is the ability to produce multi-material prints. By printing the hinge section in a flexible material a living hinge design can be produced that will last a large number of cycles. Recommended hinge specifications: 0.4 – 0.8mm thick
Post-processing
Living hinges can be annealed after printing to increase the number of cycles before failure. This can be achieved by heating up the hinge (for example by gently running a flame over the hinge until it reaches a flexible state without melting), working it back and forth for several cycles at that elevated temperature, and then leaving it in the closed position to cool down.
The effectiveness of this process will depend greatly on the material and the geometry of the hinge. For the example shown in the images above (which is printed with FDM), annealing greatly increased the number of cycles the hinge could withstand before failure.
Recommended materials
Injection-molded living hinges are made almost exclusively from polyethylene (PE) and polypropylene (PP) plastics. Both materials are flexible and soft with a relatively low melting point.
For 3D printed living hinges, materials that have a high elongation at break and good tear resistance are most suitable. The recommended materials for each process described above are summarised below.
Technology: FDM
Recommended material
Single material: Nylon 12
Multi-material: PLA or ABS for the body, and flexible TPU for the hinge
Technology: SLS
Recommended material
Nylon (PA 12 or PA 11)
Technology: Material Jetting
Recommended Material
Single material: Durable resin (PP-like)
Multi-material: Durable resin (PP-like) or Digital ABS for the body and flexible Rubber-like resin for the hinge
Rules of thumb
Living hinges made via 3D printing service in India are best suited for proof-of-concept designs before investing in expensive injection mold tooling. Living hinge geometry should have a long outer surface path and a short internal path.
Designing a reasonable Living Hinge in plastic product
Living hinges are thin sections of plastic that connect two segments of a part to keep them together and allow the part to be opened and closed. Typically these are used in containers that are used in high volume applications such as toolboxes, fish tackle boxes, CD boxes etc.
The materials used in online 3D Printing India to make a living hinge are usually a very flexible plastic such as polypropylene and polyethylene. These can flex more than a million cycles without failure.
Besides meeting the design guidelines, the hinges have to be processed properly with online 3D Printing Bangalore. The molecules have to be oriented along the hinge line for the hinge to have acceptable life.
As molded the fibres of the plastic are somewhat random in orientation. In order to orient the fibres to aid in prolonging the hinge life, some or all of the following practices should be followed:
1) The gate location should be such as to allow the plastic to flow across the hinge for maximum strength.
2) As the part comes out of the mold, it needs to be flexed a minimum of 2 times while it is still hot, for optimum strength
3) Coining is often done to give the hinge enhanced properties. The coining process compresses the hinge to a predetermined thickness. The strain induced is greater than the yield stress of the plastic. This will plastically deform the hinge (i.e. place it outside the elastic range into the plastic range). The amount of coining (compression) should be less than the ultimate stress, to keep the hinge from fracturing.
4) The finished thickness after coining should be from 0.25 to 0.5 mm (0.010 to 0.020 inch). This keeps the stress in the outer fibres from exceeding the yield strength when being flexed.
This process can also be done by heating the hinge or the coining tool to a temperature below the glass transition temperature of the plastic. This allows for easier coining and somewhat enhanced properties, as the plastic “flow” easier when being heated.